I did indeed copy the PDF into the K4PC folder and it did work. The book then showed in the Kindle. When I opened the book in the Kindle, the writing was tiny and hard to read. Can this be changed? Can you add a cover for the book in the Kindle as you can in Calibre?

If the PDF has a cover as its 1st page, K4PC will show it but you can't add a cover.

PDF is a format designed for controlling how text appears on a printed page. There are two approaches to displaying a PDF: a) like the printed page but with zoom, and b) extract text from page and "reflow" it. On your Nook (i.e. a small screen) reflow is usually best. On a PC (i.e. a big screen) zoom is more commonly used. K4PC is using zoom, so to make text bigger select the "page zoom" option at the top of the page (selector with % in it) and choose (say) "fit to width". There are many other PDF viewers for a PC and almost all of them use the zoom approach. If you want to try reflow on a PC, I suggest Sony Reader for PC.